In general, the reinforcement of metal matrices with ceramics is known in the art. Examples of ceramic materials used for reinforcement include particles, discontinuous fibers (including whiskers) and continuous fibers, as well as ceramic pre-forms. Typically, ceramic material is incorporated into a metal to produce a metal matrix composite (MMC) having improved mechanical properties as compared to the metal itself.
Some articles undergo post-formation machining (e.g., the creation of holes, threads, or other elements requiring the removal of material to provide a desired shape). Conventional MMC articles typically contain sufficient ceramic reinforcement material to make the machining impractical or at least undesirable. Typically, the presence of the ceramic material quickly wears the cutting tool away making machining of the MMC undesirable. Hence, it is preferred to produce “net-shaped” or “near net-shaped” articles that require little, or no, post-formation machining or processing. In general, techniques for making net-shaped articles are known in the art (e.g., U.S. Pat. No. 5,234,045 (Cisko) and U.S. Pat. No. 5,887,684 (Döll et al.)).
In addition, or alternatively, to the extent feasible, the ceramic reinforcement may be reduced or not placed in areas where it will interfere with machining and/or other processing such as welding. For example, metal sleeves and/or inserts may be used in conjunction with the MMC article, with the post-formation machining substantially limited to the sleeves and/or inserts. However, this construction may lead to a weak interface between the MMC casting and the metal sleeve and/or insert.
Another consideration in designing and making MMC articles is the cost of the ceramic reinforcement material itself. Although, the mechanical properties of ceramic materials such as, for example, some continuous polycrystalline alpha-alumina fibers are high compared to low-density metals such as aluminum, the cost of such ceramic oxide materials is typically substantially more than metals such as aluminum. Hence, it is desirable to minimize the amount of ceramic oxide material used, and to try to optimize placement of the ceramic oxide materials in order to maximize the properties imparted by the ceramic oxide materials.
In some embodiments, it is desirable to provide MMC articles having ceramic material in areas of high stress. In another aspect, in some embodiments, it is desirable to form net-shaped MMC articles (e.g., net-shaped, threaded MMC articles).